This invention relates to blind fasteners for securing sheet material together and, in particular, to a blind fastener that during installation is transformed into a solid shank fastener capable of withstanding high shear and tensile loads.
Blind fasteners have become particularly useful in securing sheet materials where it is impossible to access the underside of the material. Typically such fasteners are inserted into an aperture through the material and include a threaded stem for engaging the fastener. Rotation of the stem draws an expansion member into engagement with the underside of the material. Expansion may be aided by a nut body which forces the expansion member radially outwardly to engage the underside. Upon complete expansion to fully clamp the sheet material, the exterior end of the threaded stem may be broken away to eliminate any protrusions from the exterior surface.
Such fasteners have wide application in aircraft and space vehicle assembly. However, the vibrations and sonic fatigue such vehicles are subject to causes loosening of the fasteners.
Another problem with previously available blind fasteners, such as blind rivets, is that when the expansion member is drawn against the workpiece, the deformation of the expansion member causes a gap to exist between the expansion member and the stem. This gap limits the shear and tensile load capability for the fastener. An attempt to eliminate the space between the stem and the expansion member was through the use of a blind fastener arrangement wherein the stem member had an enlarged head which when drawn up through the expansion member caused the expansion member to expand only to the size of the enlarged head on the stem. The problem with this arrangement is that the footprint or surface area of the expansion member which engaged the backside of the workpiece is limited. A limited footprint also contributed to its inability to withstand high shear and tensile loads.
Other problems with previously available blind fasteners is that they do not provide a mechanism to visually determine if the fastener was installed, fails to provide a flush surface after installation, or incorporate frangible or loose pieces which could result in damage to internal damage to components of the vehicle. Other problems with previously available blind fasteners is that they require installation through the use of an expensive installation tool.
Consequently, a need exists for an improved blind fastener arrangement which can withstand high shear and tensile loads and eliminates the problems associated with previously available blind fasteners.